For those who don't know how to make this a bookmark here are the steps:
1: Open Firefox
2. Click on "Bookmarks" once so it brings up the list.
3. Right-click anywhere in the bookmarks folder and select "new bookmark"
4. When the new bookmark dialog box opens up enter the "Name" of your new bookmark (I called mine Asteroid Blast), and for the "Location" paste the following text:

(same as [Grix's] above without the spaces)
5. Click "Add" button
6. Open any website (using firefox), then click on your new bookmark to play asteroids (use arrows for propulsion, and the spacebar for "shoot") on the page and blast the text and adds off of the page.